initialization

问题 I am trying to return an instance from custom init in subclass of NSMutableURLRequest : class Request: NSMutableURLRequest { func initWith(endPoint:String, methodType:RequestType, body:RequestBody,headers: [String:String]?) { self = NSMutableURLRequest.init(url: URL.init(string: endPoint, relativeTo: URL?)) //return request } } But compiler does not allow to do the same and i get the error "Cannot assign to value: 'self' is immutable". What is the correct way to go about this and why does the

问题 In the following code , I am initializing a reference variable with a literal. class ABC { public: const int& a; ABC():a(43) { } void newfoo() { printf("NEWFOO %d",a); } }; int main() { ABC obj; obj.newfoo(); } The output of this program is NEWFOO 32767 which seems illogical when I know that the following code works just fine. int main() { const int& b=3; printf("%d",b); } What is happening here ? If compiler declares some temp variable during initializing of the reference variable , then isn

问题 #include <ios> #include <iostream> #include <map> using namespace std; int main() { ios_base::sync_with_stdio(false); map<int, int> v; int i; int t; while (cin >> i) { v[i] = t++; } auto mi = i; auto mt = t; for (const auto p : v) { if (p.second < mt) { mi = p.first; mt = p.second; } } cout << mi << '\n'; return 0; } The abovementioned program makes heavy use of an uninitialized variable t , but GCC does not report it with -Wall or -Wuninitialized. Why is it so? It is worth noting that Clang

问题 To a WinForms control, I would like to add a handler after the container has initialized the control (or even better, after the parent has initialized all contained controls). Reason: The custom control has an option to trigger an action automatically. It should also trigger when this option is first enabled. However, at this point, other properties (like event handlers) are not yet wired up correctly, and thus the effect is not as expected. Is this possible? [edit] I understand that this

问题 Variables of built-in types can be value-initialized like this: int var = int(); this way I get the default value of int without hardcoding the zero in my code. However if I try to do similar stuff for a pointer: int* ptr = int*(); the compiler (Visual C++ 10) refuses to compile that (says type int unexpected ). How do I value-initialize a pointer in similar manner? 回答1: How do I value-initialize a Type* pointer using Type()-like syntax? You cannot. The syntax T() is defined in 5.2.3/1,2 (C+

问题 I have this snippet of code which seems to work well: class foo{/* some member variables and functions*/}; void do_somthing(foo x={}){} int main(){ do_somthing(); } I used to use void do_somthing(foo x=foo()){} to default the x argument but I see this way ={} in some book or online example(can not remember). Is it totally ok to use it? Is there any difference between the two methods? 回答1: foo x=foo() is copy initialization, Initializes an object from another object and foo() is value

问题 I have this snippet of code which seems to work well: class foo{/* some member variables and functions*/}; void do_somthing(foo x={}){} int main(){ do_somthing(); } I used to use void do_somthing(foo x=foo()){} to default the x argument but I see this way ={} in some book or online example(can not remember). Is it totally ok to use it? Is there any difference between the two methods? 回答1: foo x=foo() is copy initialization, Initializes an object from another object and foo() is value

问题 [basic.scope.pdecl]/1 of the C++20 standard draft had the following (non-normative) example in a note (partial quote from before the merge of pull request 3580, see answer to this question): unsigned char x = x; [...] x is initialized with its own (indeterminate) value. Does this actually have well-defined behavior in C++20? Generally the self-initialization of the form T x = x; has undefined behavior by virtue of x 's value being indeterminate before initialization is completed. Evaluating

问题 struct some_struct{ int a; }; some_struct n = {}; n.a will be 0 after this; I know this braces form of initialization is inherited from C and is supported for compatibility with C programs, but this only compiles with C++, not with the C compiler. I'm using Visual C++ 2005. In C this type of initialization struct some_struct n = {0}; is correct and will zero-initialize all members of a structure. Is the empty pair of braces form of initialization standard? I first saw this form of

问题 struct some_struct{ int a; }; some_struct n = {}; n.a will be 0 after this; I know this braces form of initialization is inherited from C and is supported for compatibility with C programs, but this only compiles with C++, not with the C compiler. I'm using Visual C++ 2005. In C this type of initialization struct some_struct n = {0}; is correct and will zero-initialize all members of a structure. Is the empty pair of braces form of initialization standard? I first saw this form of